1. Field of the Invention
The application relates to a method and a related communication device used in a wireless communication system and related communication device, and more particularly, to a method for reference signal pattern allocation and a related communication device in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNodeBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
A long term evolution-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system, considering relaying for cost-effective throughput enhancement and coverage extension. The LTE-A system includes all of the features of the LTE system and several new ones, the most important of which are: carrier aggregation, enhanced multi-antenna support and relaying. The LTE system provides extensive support for deployment in spectrum allocations of various characteristics, with transmission bandwidths ranging from 1.4 MHz up to 20 MHz. In the LTE-A system, the transmission bandwidth can be further extended with carrier aggregation wherein multiple component carriers are aggregated and jointly used for transmission to/from a signal UE. In general, up to five component carriers can be aggregated, allowing for transmission bandwidth up to 100 MHz.
In order to provide better performance in LTE-A system (e.g. higher spectrum efficiency), multiple transmit antennas must be supported in the LTE-A. Demodulation reference signal (DM RS) design will influence channel estimation accuracy and eventually determine reliability and throughput. The DMRS is pre-coded and send through multiple antennas. The pre-coding may enhance reception performance to a mobile device or user and improve channel estimation performance.
Reference signals targeting PDSCH demodulation can be UE-specific, i.e., the physical downlink shared channel (PDSCH) and the demodulation reference signals intended for a specific UE are subject to the same precoding operation. Present only in resource blocks and layers scheduled by the eNodeB for transmission. Reference signals transmitted on different layers are mutually orthogonal at the eNodeB.
The design principle for the demodulation reference signals is an extension to multiple layers of the concept of 3GPP Rel-8 UE-specific reference signals used for beamforming. Complementary use of 3GPP Rel-8 cell-specific reference signals by the UE is not precluded.
Please further refer to FIG. 1, which illustrates a DM RS OCC pattern in the prior art. The DM RS pattern for rank 1-4 can be hybrid code division multiplexing (CDM) with frequency division multiplexing (FDM). Length-2 OCC might be used in time domain (CDM-T) with Walsh sequences {1, 1} and {1, −1}. Forward and reverse mappings take in turn from higher frequency to lower frequency. Common reference signal (CRS) is also placed on the frequency and time domain but it may not be pre-coded. The eNodeB may multiply the OCC with the associated DM RS signal and transmit through the associated antenna or antenna port.
Please refer to FIG. 2, which illustrates an exemplary DM RS OCC mapping with CDM for layers 1 and 2 in the prior art. This mapping ignores the CRS and the grey portion (non-slash part) shown in FIG. 1. The mapping considers two physical resource blocks (PRBs), each PRB is composed of 7 OFDM symbols in time domain and 12 subcarriers in frequency domain, and these two PRBs are contiguous in time domain.
For rank 1 and 2, the same DM RS structure (including patterns, spreading and scrambling) as in 3GPP LTE Rel-8/9 is used, as illustrated in FIG. 1. For rank 2, DM RS for 1st layer and that for 2nd layer are multiplexed by means of code division multiplexing (CDM) by using orthogonal cover code (OCC) over two consecutive resource elements (blue) in time domain.
For rank 3 and 4, the DM-RS pattern is illustrated in FIG. 1. DM-RS for 1st layer and that for 2nd layer are multiplexed by means of CDM by using OCC over two consecutive resource elements (blue) in time domain. DM RS for 3rd layer and that for 4th layer are multiplexed by means of CDM by using OCC over two consecutive resource elements (green) in time domain. DM-RS for 1st and 2nd layers and that for 3rd and 4th layers are multiplexed by means of frequency division multiplexing (FDM). The DM RS signals can apply a length-31 Gold sequence.
FIG. 3 is extended from FIG. 2 and considers the OCC mapping with Walsh codes {1, 1} and {1, −1}. This figure further considers frequency domain contiguous two PRBs. If there are two contiguous PRBs for a user, the successive PRB will apply reversed OCC mapping the previous PRB. It introduces 2-D orthogonality among layers and peak power randomization.
FIG. 4 shows DM-RS pattern for ranks 5-8. Hybrid CDM+FDM DM-RS patterns are used. Length-4 OCC in time domain (CDM-T) is used with Walsh sequences {1, 1, 1, 1}, {1, −1, 1, −1}, {1, 1, −1, −1}, {1, −1, −1, 1}. The design criteria for length-4 mappings are backward compatibility, 2-D orthogonality, peak power randomization. For backward compatibility with agreed mapping scheme for up to rank 2, OCC sequences for higher rank transmissions are desired to be a superset of that for rank transmissions. The eNodeB may multiply the OCC with the associated DM RS signal and transmit through the associated antenna or antenna port.
For 2-D orthogonality in time and frequency domains, time-domain orthogonality means subcarriers a, b, c and d are mapped to four resource elements (REs) in time domain and frequency-domain orthogonality means subcarriers a, b, c, d are mapped to closet four resource elements (REs) in frequency domain. For peak power randomization, it is achieved by time/frequency variation of OCC mapping within OFDMA symbols and subcarriers a, b, c, d are mapped such that subcarriers a, b, c, d are included in frequency domain. FIG. 5 illustrates an exemplary mapping for contiguous two PRBs. This maintains 2-D orthogonality while randomizing peak power.
These mappings depict that contiguous two PRBs will apply two patterns to achieve some of these three features: backward compatibility, 2-D orthogonality, peak power randomization. If a mobile station or user receives contiguous two PRBs, it may follow the order pattern A and pattern B. However, if a user receives multiple PRBs distributed on frequency domain, the rule may be different because these PRBs are not contiguous.